Multilayer cholesteric pigments

ABSTRACT

A platelet-shaped cholesteric multilayer pigment which comprises the layer sequence A1/B/A2whereA1 and A2 are identical or different and each comprise at least one cholesteric layer, andB is at least one interlayer which separates the layers A1 and A2 from one another and which absorbs all or some of the light transmitted by the layers A1 and A2,processes for its preparation, and its use are all described.

The invention relates to multilayer cholesteric pigments, to processesfor their preparation and to their use.

When substances exhibiting shape anisotropy are heated it is possiblefor liquid-crystalline phases known as mesophases to occur. Theindividual phases differ in the spatial arrangement of the centers ofmass of the molecules, on the one hand, and by the arrangement of themolecules with respect to the long axes, on the other hand (G. W. Gray,P. A. Winsor, Liquid Crystals and Plastic Crystals, Ellis HorwoodLimited, Chichester 1974). The nematic liquid-crystalline phase isdistinguished by parallel orientation of the long axes of the molecules(one-dimensional order state). Provided that the molecules forming thenematic phase are chiral, the result is a chiral nematic (cholesteric)phase in which the long axes of the molecules form a helicalsuperstructure perpendicular thereto (H. Baessler, FestkörperproblemeXI, 1971). The chiral moiety may be present in the liquid-crystallinemolecule itself or else may be added as a dopant to the nematic phase,inducing the chiral nematic phase. This phenomenon was firstinvestigated on cholesterol derivatives (eg. H. Baessler, M. M. Labes,J. Chem. Phys. 52, (1970) 631).

The chiral nematic phase has special optical properties: a high opticalrotation and a pronounced circular dichroism resulting from selectivereflection of circularly polarized light within the chiral nematiclayer. Depending on the pitch of the helical. superstructure it ispossible to produce different colors which can appear differentdepending on the angle of view. The pitch of the helical superstructuredepends in turn on the twisting power of the chiral component. In thiscase, it is possible, in particular by altering the concentration of achiral dopant, to vary the pitch and hence the wavelength range of theselectively reflected light of a chiral nematic layer. Chiral nematicsystems of this type have interesting possibilities for practical use.

Cholesteric special-effect pigments and compositions comprising suchpigments are known.

EP-A-686 674 and its parent DE-A-44 16 191 describe interferencepigments comprising molecules which are fixed in a cholestericarrangement; the pigments feature a platelet-shaped structure and have,as one example demonstrates, a layer thickness of 7 μm. The pigments areprepared by applying highly viscous LC material to a substrate, thesubstrate being conveyed at a speed of about 2 m/min below a fixeddoctor blade. By this means the liquid-crystalline molecules areoriented.

WO 97/30136 describes cholesteric polymer platelets which are obtainablefrom a chiral polymerizable mesogenic material. The platelets can beused as effect pigments. They are single-layered and have a preferredthickness of from 4 to 10 μm.

DE-A-196 39 179 discloses transparent compositions which comprisepigments whose coloredness is dependent on the viewing angle. Thecompositions are said to contain absorbtive colorants in an amount suchthat the angle-dependent color effect which occurs in the specular-angleconfiguration is intensified without any definite adverse effect on thetransparency of the composition under all other angular configurations.The pigments are prepared as described in the abovementioned EP-A-686674.

DE-A-196 39 229 discloses a composition in which at least one matrix,containing pigments whose coloredness is dependent on the viewing angle,is present in at least one further matrix in a two- orthree-dimensionally structured format in the form of structuralelements. These matrices 1 and 2 are non-identical, or containnon-identical pigments in identical concentrations. The pigments areprepared as described in the abovementioned EP-A-686 674.

DE-A-196 39 165 describes a method of obtaining new color effects bymeans of pigments whose coloredness depends on the viewing angle, in amatrix, where following their incorporation into the matrix the pigmentsare subjected to three- and/or two-dimensionally selective tilting inthe matrix. The pigments are tilted preferably by means of differentlydirected movement of the pigments in the matrix or by using pigmentsdiffering in their concentration in the matrix. The pigments areprepared as described in the abovementioned EP-A-686 674.

DE-A-195 02 413 describes a pigment whose coloredness is dependent onthe viewing angle and which has been obtained by three-dimensionalcrosslinking of oriented substances of liquid-crystalline structure witha chiral phase. In order to render such pigment colorfast with respectto elevated temperatures, it is proposed to conduct crosslinking in thepresence of at least one additional, color-neutral compound whichcontains at least two crosslinkable double bonds. The uncrosslinkedcholesteric mixture is applied by knife coating onto a film, forexample. No details are given regarding the thickness of the layerapplied by knife coating.

U.S. Pat. No. 5,364,557 describes cholesteric inks, for drawing andprinting, which comprise platelet-shaped or flakelike cholestericpigments. The pigments are prepared by coating a conveyor belt with acholesteric melt and then smoothing and aligning the cholesteric film bymeans of a blade. The pigments can comprise two layers of differenthandedness or two layers of identical handedness with a further layerbetween them which inverts the direction of rotation of circularlypolarized light. A light-absorbing layer is not described as a possibleconstituent of a layer pigment, especially since such a layer would workagainst the desired effect of 100% reflection of light.

U.S. Pat. No. 5,599,412 discloses a preparation process and apparatusfor preparing cholesteric inks for writing and printing. Application andalignment of the melted polymer take place as described in theabove-cited U.S. Pat. No. 5,364,557.

DE-A-44 18 076 describes effect coating materials and effect finisheswhich comprise interference pigments formed from esterified celluloseethers. Using these interference pigments it is possible, it is said, toproduce, in the case of the coating material, color changes which aredependent on the direction of incident light and on the direction ofviewing, or to produce particularly intense hues of hitherto unknownbrilliance on the article coated with such a material. The pigments areprepared by comminuting cholesteric layers which after curing are saidto have a thickness of 5 to 200 μm. The layers are applied, for exampleby knife coating. In the examples, a layer thickness of approximately 10μm is stated.

DE-A-196 297 61 describes cosmetic or pharmaceutical formulations whichcomprise pigments whose coloredness is dependent on the viewing angle.The pigments comprise at least one oriented crosslinked substance with aliquid-crystalline structure and with a chiral phase. The pigments areplateletlike in form and have a thickness of 1 to 20 μn. Preparation ofthe pigments is as described in the abovementioned EP-A-686 674,although the layer thickness is said to be 5 μm rather than 7 μm.

Known from WO 96/02597 is a method of coating or printing substrateswith a composition that comprises a chiral or achiral,liquid-crystalline monomer and a non-liquid-crystalline, chiralcompound. The liquid-crystalline monomers are preferably photochemicallypolymerizable bisacrylates. To achieve the uniform orientation of thecholesteric phase, which is required for the development of the desiredoptical properties, and to achieve it even on large surfaces of complexform, it is necessary to add a polymeric binder. The layers, about whosethickness nothing is stated, are prepared via various printingtechniques or by spraying.

DE-A-196 02 795 discloses a process for preparing pigment particles. Thepigment particles have a substantially uniform, defined form and size,being prepared either by polymerizing a polymerizable mixture inuniformly dimensioned recesses or by initial shaping, by means of aprinting process, and subsequent polymerization. The layer thickness ofthe pigments is not mentioned.

DE-A-196 02 848 describes a process for preparing pigments which employsa polymerizable mixture necessarily comprising, inter alia, a polymericbinder and/or monomeric compounds which can be converted bypolymerization into a polymeric binder, and/or comprising a dispersingauxiliary. These auxiliaries are said to bring about a considerableimprovement in the flow viscosity.

DE-A-42 40 743 discloses pigments whose coloredness is dependent on theviewing angle. The pigments are preferably prepared fromthree-dimensionally crosslinkable polyorganosiloxanes, theliquid-crystal mass being knife-coated onto a metal, plastic or glasssubstrate, crosslinked thermally or photochemically, and then thecrosslinking product being detached from the substrate. The pigmentspreferably have a thickness of from 5 to 50 μm.

EP-B-383 376 describes liquid-crystal pigments comprisingplatelet-shaped carrier particles some of which at least are coated withliquid-crystalline material. Coating takes place by dispersing theplatelet-shaped particles in a solvent in which liquid-crystallinematerial is dissolved, and then precipitating at least some of theliquid-crystalline material onto the particles. In the course of thisthe platelet-shaped carrier particles become fully or partly envelopedby the cholesteric. Uniform cholesteric layers arranged exactly parallelto the middle layer cannot be prepared by this process. The pigments areapparently not fully hiding, since they are said to be appliedpreferably to black surfaces.

DE-A-196 19 973 outlines, in a non-imitable manner, an idea for two- orthree-layer platelet-shaped interference pigments. The pigments areintended to have at least one layer which consists of liquid-crystallinepolymers whose mesogens are at least approximately in chiral-nematicand/or smectic and/or cholesteric order. Also provided in theinterference pigments is a light-absorbing layer which is absorbent forat least part of the visible spectrum of light. The pigments are to beobtainable by knife coating, rolling or spray application to a smoothsubstrate, curing of the thin film thus produced, application of thelight-absorbing layer, curing of this light-absorbing layer, optionalapplication and curing of a further film which coincides with the firstfilm in its composition and layer thickness, and removal and comminutionof the cured layer assembly. Specific pigments, however, are notdisclosed. As far as the material composition of the pigments isconcerned, all that is said is that “liquid-crystalline main-chain orside-chain polymers or mixtures thereof, liquid-crystalline oligomers oroligomer mixtures, or liquid-crystalline monomers or monomer mixtures,[come] into consideration” as liquid-crystalline polymers. There are noexamples regarding the preparation of the pigments or thepigment-containing coating formulations. The disclosure content ofDE-A-196 19 973 is therefore limited to purely theoretical discussionsof the idea of two- or three-layer pigments. Consequently, no technicalteaching is provided that is imitable by the skilled worker.

WO 94/22976 and its parent GB-A 2276883 describe two-layer cholestericpigments based on two different polyorganosiloxanes from the companyWacker. The pigments are prepared in an extremely complex manner byseparate coating of two previously nylon-coated glass plates withsolutions of the abovementioned liquid crystals; rubbing of eachliquid-crystal layer in order to orient it; attachment of thermallydeformable spacers to the glass plates; placing of the glass platestogether with their cholesteric layers facing one another, and unitingof the cholesteric layers by thermal deformation of the spacers atelevated temperature in a vacuum, and also crosslinking of the unitedcholesteric layers. The film thus obtainable is said, like the pigmentsobtainable from it by milling, to have a thickness of approximately 10μm. Despite the prior coating of the glass plates with nylon, detachmentof the film from the glass plates is apparently incomplete, so thatresidues of the film have to be scraped off in order to obtain thepigments from the plates, which makes the preparation of the pigmentseven more complex. The idea of three-layer pigments is merely outlined.These pigments cannot be prepared by the preparation process describedfor two-layer pigments. Wo 94/22976 and its parent GB-A-2276883therefore provide no technical teaching which is imitable by the skilledworker and which would in any way provide three-layer pigments. Thedisclosure content is limited to purely theoretical discussions of thestructure of three-layer pigments.

In order to absorb the transmitting wavelength range, the priorinterference pigments must either contain additional pigments in thecholesteric matrix or be applied to a colored background. When foreignpigments are incorporated into the liquid-crystalline mass it isdisadvantageous that a considerable portion of the reflecting wavelengthrange is absorbed or scattered by absorption and scattered light, sothat the special advantage of the interference pigments on a cholestericbasis is largely removed. The same problem occurs if cholestericpigments are mixed with absorbing pigments into coating formulations.Reflections which disrupt the perceived color can only be avoided if theabsorbing pigment is dispersed very finely into the cholesteric matrix.From general experience this is only the case if the pigment isdispersed using additives tailored specifically to the pigment surface.These compounds, such as fatty acids, salts of fatty acids, soyalecithins or phosphoric esters, however, interfere with the developmentof the helical orientation and this prevents optimum color development.If, on the other hand, absorption takes place over a colored underlayer,the background must be of uniformly high quality in order to provide thedesired overall impression of the effect coating. Consequently,considerable effort has to be expended on pretreating the background. Anideal background for maximum brilliance would have to be black or havespecular gloss, which in the case of car bodies, for example, would beextremely difficult to realize.

It is an object of the present invention to provide special-effectpigments which no longer have the above-described disadvantages of theprior art.

We have found that this object is achieved by a multilayer pigment whichcomprises at least one layer which absorbs all or some transmittedlight, installed between cholesteric layers.

The present invention therefore provides a platelet-shaped cholestericmultilayer pigment which comprises the layer sequence A¹/B/A² where

A¹ and A² are identical or different and each comprise at least onecholesteric layer, and

B is at least one interlayer which separates the layers A¹ and A² fromone another and which absorbs all or some of the light transmitted bythe layers A¹ and A².

The multilayer pigment of the invention offers a range of surprisingadvantages:

a) B can be made completely opaque (completely absorbing transmittedlight) so that, with a sufficient level of pigmentation, the perceivedcolor of the pigment is completely independent of the substrate andthere is no need for the hitherto customary complex and costly tailoringof the substrate for transparent interference pigments. Self-opacifyingcholesteric special-effect pigments are therefore provided for the firsttime.

b) The color of B can be varied, thereby providing a further controlvariable for the perceived color of the overall pigment.

c) The brightness of the overall pigment can additionally be adjusted byvarying the gloss and/or roughness of B.

d) B can be tailored to the specific application in order to adjust thehardness and/or flexibility of the overall pigment.

e) B can be electrically conductive and is able thereby to impartelectrical conductivity to the overall pigment without, in so doing,impairing the quality of the cholesteric layers.

f) A¹, B and A² are of uniform thickness and are stacked atop oneanother in parallel, hence forming a kind of sandwich structure whichconsiderably increases the brightness of the pigment. This also bringsabout an improved perceived color in comparison with pigments having anall-round coating, since all of the cholesteric molecules of one layerhave the same alignment.

g) The perceived color of the pigment is largely independent of externalstimuli; in other words, it is stable over a wide range of temperatureand pressure.

The top and the bottom cholesteric layer A¹ and A², respectively, of thepigment of the invention possess identical or different opticalproperties. They may, in particular, reflect light of the same or adifferent wavelength; in other words, they may be of the same or of adifferent color. In the latter case it is possible to achieveparticularly interesting color effects. A¹ and A² are preferablydifferent in their handedness, so that, for example, A¹ providesreflection of light of a certain wavelength in a left-handedlycircular-polarized manner while A² reflects light of the same wavelengthin a right-handedly circular-polarized manner. Advantageously,therefore, a paint comprising pigments of the invention in thispreferred embodiment, for example, appears particularly bright since A¹and A² in statistical distribution in the coat of paint face theincident light so that the coating reflects both right-handedly andleft-handedly circular-polarized light of a certain wavelength, whereasa paint containing only pigments having just one cholesteric layer orhaving a plurality of cholesteric layers of the same handednesstransmits either the left- or right-handedly circular-polarized light.

A¹ and A² can also be identical or different in terms of theirmechanical properties. They may, for example, differ in thickness orbrittleness.

B preferably comprises at least one organic or inorganic absorptionpigment, preferably bound into a binder matrix. The absorption pigmentcan be a white, color or—preferably—black pigment. Examples of suitableinorganic absorption pigments are titanium dioxide, Al₂O₃, bariumsulfate, strontium sulfate, zinc oxide, zinc phosphates, black ironoxide, lead chromate, strontium chromate, barium chromate and also metalpigments such as aluminum powder or bronze powder.

Examples of suitable organic absorption pigments are azo pigments, metalcomplex pigments, such as azo- and azomethine-metal complexes,isoindolinone and isoindoline pigments, phthalocyanine pigments,quinacridone pigments, perinone and perylene pigments, anthraquinonepigments, diketopyrrolopyrrole pigments, thioindigo pigments, dioxazinepigments, triphenylmethane pigments, quinophthalone pigments andfluorescent pigments.

Particularly suitable absorption pigments are fine such pigments havingan average particle size of from 0.01 to 1 μm, preferably from 0.01 to0.1 μm.

It is preferably possible to employ graphite pigments or various gradesof carbon black, especially readily dispersible pigment-grade carbonblacks having a specific surface area of from 30 to 150 m²/g (BETmethod) and an absorption capacity of from 50 to 100 ml of dibutylphthalate/100 g (DBP number).

Particularly preferred absorption pigments are those which impartmagnetic properties to the layer B which absorbs transmitted light.Suitable examples are γ-Fe₂O₃, Fe₃O₄, CrO₂ or ferromagnetic: metalpigments, such as Fe, Fe—Cu and Fe—Ni—Co alloys, for example. With thesepigments it is possible to produce highly lustrous black intermediatelayers.

Pigments whose absorbing layer is magnetic can, advantageously, be givenan arbitrary orientation by application of a magnetic field. In this wayit is possible, for instance, to prevent individual pigment flakesprojecting from the others, which results in the scattering of lesslight and an improvement in the perceived color. All of the flakes canbe oriented together in a defined angle. It is also possible to generatefull-area patterns for obtaining new color effects, or partial patternsfor optical emphasis of letters or structures. The magnetic cholestericpigments of the invention can also be employed with advantage in aliquid matrix, for example in LCDs, in which they alter their directionand therefore their perceived color when a magnetic field is applied.

The absorption pigments are preferably bound into an organic bindermatrix. Suitable binders are the customary coatings systems. Suitablesystems are preferably radiation-curable systems comprising reactive,crosslinkable groups, such as acrylic, methacrylic, α-chloroacrylic,vinyl, vinyl ether, epoxy, cyanate, isocyanate or isothiocyanate groups.

Other binders which can be employed are monomeric agents and mixturesthereof with polymeric binders. Preferred monomeric agents are thosewhich have two or more crosslinkable groups, such as acrylic,methacrylic, α-chloroacrylic, vinyl, vinyl ether, epoxy, cyanate,isocyanate or isothiocyanate groups. Particular preference is given toacrylic, methacrylic or vinyl ether groups. Examples of monomeric agentshaving two crosslinkable groups are the diacrylates, the divinyl ethersor the dimethacrylates of diols such as propanediol, butanediol,hexanediol, ethylene glycol, diethylene glycol, triethylene glycol ortetrapropylene glycol, for example.

Examples of monomeric agents having three crosslinkable groups are thetriacrylates, the trivinyl ethers or the trimethacrylates of triols suchas trimethylolpropane, ethoxylated trimethylolpropane having 1 to 20ethylene oxide units, propoxylated trimethylolpropane having 1 to 20propylene oxide units, and mixed ethoxylated and propoxylatedtrimethylolpropane in which the sum of ethylene oxide and propyleneoxide units is from 1 to 20. Examples of monomeric agents having threecrosslinkable groups are also the triacrylates the trivinyl ethers orthe trimethacrylates of glycerol, ethoxylated glycerol having 1 to 20ethylene oxide units, propoxylated glycerol having 1 to 20 propyleneoxide units, and mixed ethoxylated and propoxylated glycerol in whichthe sum of ethylene oxide and propylene oxide units is from 1 to 20.

Examples of monomeric agents having four crosslinkable groups are thetetraacrylates, the tetravinyl ethers or the tetramethacrylates oftetraols such as bis-trimethylolpropane, ethoxylatedbis-trimethylolpropane having 1 to 20 ethylene oxide units, propoxylatedbis-trimethylolpropane having 1 to 20 propylene oxide units, and mixedethoxylated and propoxylated bis-trimethylolpropane, in which the sum ofethylene oxide and propylene oxide units is from 1 to 20. Furtherexamples of monomeric agents having four crosslinkable groups are thetetraacrylates, the tetravinyl ethers or the tetramethacrylates oftetraols such as pentaerythritol, ethoxylated pentaerythritol having 1to 20 ethylene oxide units, propoxylated pentaerythritol having 1 to 20propylene oxide units, and mixed ethoxylated and propoxylatedpentaerythritol in which the sum of ethylene oxide and propylene oxideunits is from 1 to 20.

To enhance the reactivity in the course of crosslinking orpolymerization in air it is possible for the binders and the monomericagents to include from 0.1 to 10% of a primary or secondary amine.Examples of suitable amines are ethanolamine, diethanolamine ordibutylamine.

With particular preference, the absorption pigments of the layer B arebound into a binder matrix which comprises the cholesteric mixturesdescribed further below as constituents of the layers A¹ and A². Withvery particular preference, the binder matrix of the layer B comprisesthe same cholesteric mixtures as the layers A¹ and A².

The absorption pigment formulation can be prepared by the customarydispersion techniques, which are known in the art, and using diluents,dispersants, photoinitiators and, if desired, further additives.

Diluents which can be used are water or organic liquids or mixturesthereof, preference being given to organic liquids. Particularlypreferred organic liquids are those having a boiling point of below 140° C., especially ethers such as tetrahydrofuran, ketones such as ethylmethyl ketone and esters such as butyl acetate.

Dispersants which can be used are low molecular mass dispersants such asstearic acid, for example, or else polymeric dispersants. Suitablepolymeric dispersants or dispersing resins are known to the skilledworker. Particular mention may be made of sulfonate-, phosphate-,phosphonate- or carboxyl-functional polyurethanes, carboxyl-functionalvinyl chloride copolymers, polyimine polyesters or polyether acrylateswith or without incorporated functional groups.

For the preparation of crosslinkable or polymerizable absorption pigmentformulations it is possible to use the photoinitiators customary forphotochemical polymerization, examples being the photoinitiators listedbelow for the photochemical polymerization of the cholesteric mixtures.

The thickness of each individual cholesteric layer of A¹ or A² ispreferably from about 0.5 to 20 μm, in particular from about 1 to 10 μmand, with particular preference, from about 2 to 4 μm. The thickness ofeach individual layer of B is from about 0.2 to 5 μm, in particular fromabout 0.5 to 3 μn. The diameter of the pigments of the invention is fromabout 5 to 500 μm, in particular from about 10 to 100 μm and, withparticular preference, from about 10 to 30 μm. In general the pigmentdiameter is approximately 5 times the pigment thickness.

A¹ and A² of the pigments of the invention preferably comprisecholesteric mixtures selected from

a) at least one cholesteric, polymerizable monomer;

b) at least one achiral, nematic, polymerizable monomer and one chiralcompound;

c) at least one cholesteric, crosslinkable oligomer or polymer; or

d) a cholesteric polymer in a polymerizable diluent;

e) at least one cholesteric polymer whose cholesteric phase can befrozen in by rapid cooling to below the glass transition temperature,

in the cured state.

Curing fixes the uniform orientation of the cholesteric molecules in thecholesteric layer.

Preferred monomers of group a) are described in DE-A-196 02 848, thefull content of which is incorporated herein by reference. Inparticular, the monomers a) embrace at least one chiral,liquid-crystalline, polymerizable monomer of the formula I

where

Z¹ is a polymerizable group or a radical which carries a polymerizablegroup,

Y¹, Y², Y³ independently are chemical bonds, oxygen, sulfur, —CO—O—,—O—CO—, —O—CO—O—, —CO—N(R)— or —N(R)—CO—,

A¹ is a spacer,

M¹ is a mesogenic group,

X is an n-valent chiral radical,

R is hydrogen or C₁-C₄-alkyl,

n is 1 to 6,

and Z¹, Y¹, Y², Y³, A¹ and M¹ can be identical or different if n isgreater than 1.

Preferred radicals Z¹ are:

CH₂═CH—, CH≡C—,

 —N═C═O, —N═C═S, —O—C≡N, —COOH, —OH or —NH₂,

where each R can be identical or different and is hydrogen orC₁-C₄-alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl,isobutyl, sec-butyl or tert-butyl. Of the reactive polymerizable groups,the cyanates are able to trimerize spontaneously to cyanurates and aretherefore preferred. Polymerization of the other groups indicatedrequires further compounds having complementary reactive groups.Isocyanates, for example, are able to polymerize with alcohols to giveurethanes and with amines to give urea derivatives. Similar commentsapply to thiiranes and aziridines. Carboxyl groups can be condensed togive polyesters and polyamides. The maleimido group is particularlysuitable for free-radical copolymerization with olefinic compounds suchas styrene. Said complementary reactive groups can be present either ina second compound of the invention, which is mixed with the first, orcan be incorporated into the polymeric network by means of auxiliarycompounds containing 2 or more such complementary groups.

Particularly preferred groups Z¹-Y¹ are acrylate and methacrylate. Y¹-Y³can be as defined above, the term a chemical bond meaning a singlecovalent bond.

Suitable spacers A¹ are all groups known for this purpose. The spacerscontain generally 1 or more, for example from 2 to 30, preferably 1 to12 or 2 to 12 carbons and consist of linear aliphatic groups. They maybe interrupted in the chain by nonadjacent O, S, NH or NCH₃, forexample. Other suitable substituents for the spacer chain are fluorine,chlorine, bromine, cyano, methyl and ethyl.

Examples of representative spacers are:

—(CH₂)_(p)—, —(CH₂CH₂O)_(m)CH₂CH₂—, —CH₂CH₂SCH₂CH₂—, —CH₂CH₂NHCH₂CH₂—,

where

m is 1 to 3 and

p is 1 to 12.

The mesogenic group M¹ preferably has the structure

(T—Y⁸)_(s)—T

where Y⁸ is a bridge in accordance with one of the definitions of Y¹, sis 1 to 3 and T is identical or different at each occurrence and is adivalent isocycloaliphatic, heterocycloaliphatic, isoaromatic orheteroaromatic radical.

The radicals T can also be ring systems substituted by C₁-C₄-alkyl,fluorine, chlorine, bromine, cyano, hydroxyl or nitro. Preferredradicals T are:

Particular preference is given to the following mesogenic groups

M¹:

Of the chiral radicals X of the compounds of the formula I particularpreference is given, partly on account of their availability, to thosederived from sugars, from binaphthyl or biphenyl derivatives and fromoptically active glycols, dialcohols or amino acids. In the case of thesugars, particular mention should be made of pentoses and hexoses andderivatives thereof.

Examples of radicals X are the following structures, the lines at theend in each case denoting the free valences.

Particular preference is given to

Also suitable, furthermore, are chiral groups having the followingstructures:

Further examples are set out in the German Application P 43 42 280.2.

n is preferably 2.

As preferred monomers of group b), the cholesteric mixture in theprocess of the invention includes at least one achiralliquid-crystalline polymerizable monomer of the formula II

Z²—Y⁴—A²—Y⁵—M²—Y⁶—A³—Y⁷—Z³)_(n)  (II)

where

Z², Z³ are identical or different polymerizable groups or radicals whichcontain a polymerizable group,

n is 0 or 1, Y ⁴, Y⁵, Y⁶, Y⁷ independently are chemical bonds, oxygen,sulfur, —CO—O—,—O—CO—O—CO—O—, —CO—N(R)— or —N(R)—CO—,

A², A³ are identical or different spacers and

M² is a mesogenic group.

The polymerizable groups, bridges Y⁴ to Y⁷, the spacers and themesogenic groups are subject to the same preferences as thecorresponding variables of the formula I.

In addition, the mixture of group b) includes a chiral compound. Thechiral compound brings about the twisting of the achiralliquid-crystalline phase to form a cholesteric phase. In this context,the extent of twisting depends on the twisting power of the chiraldopant and on its concentration. Consequently, therefore, the pitch ofthe helix and, in turn, the interference color depend on theconcentration of the chiral dopant. As a result, it is not possible toindicate a generally valid concentration range for the dopant. Thedopant is added in the amount at which the desired color effect isproduced.

Preferred chiral compounds are those of the formula Ia

where Z¹, Y¹, Y², Y³, A¹, X and n are as defined above and M^(a) is adivalent radical which comprises at least one heterocyclic or isocyclicring system.

The moiety M^(a) here is similar to the mesogenic groups described,since in this way particularly good compatibility with theliquid-crystalline compound is achieved. Ma, however, need not bemesogenic, since the compound Ia is intended merely by means of itschiral structure to bring about the appropriate twisting of theliquid-crystalline phase. Preferred ring systems present in M^(a) arethe abovementioned structures T, preferred structures M^(a) being thoseof the abovementioned formula (T—Y⁸)_(s)—T. Further monomers and chiralcompounds of group b) are described in WO 97/00600 and its parentDE-A-195 324 08, the full content of which is expressly incorporatedherein by reference.

Preferred polymers of group c) are cholesteric cellulose derivatives asdescribed in DE-A-197 136 38, especially cholesteric mixed esters of

(VI) hydroxyalkyl ethers of cellulose with

(VII) saturated, aliphatic or aromatic carboxylic acids and

(VIII) unsaturated mono- or dicarboxylic acids.

Particular preference is given to mixed esters in which the hydroxyalkylradicals of component VI that are attached by way of ether functions arestraight-chain or branched C₂-C₁₀-hydroxyalkyl radicals, especiallyhydroxypropyl and/or hydroxyethyl radicals. Component VI of the mixedesters of the invention preferably has a molecular weight of from about500 to about 1 million. Preferably, the anhydroglucose units of thecellulose are etherified with hydroxyalkyl radicals in an average molardegree of substitution of from 2 to 4. The hydroxyalkyl groups in thecellulose can be identical or different. Up to 50% of them can also bereplaced by alkyl groups (especially C₁-C₁₀-alkyl groups). One exampleof such a compound is hydroxypropylmethylcellulose.

Compounds which can be used as component VII of the mixed esters thatare employable are straight-chain aliphatic C₁-C₁₀ carboxylic acids,especially C₂-C₆ carboxylic acids, branched aliphatic C₄-C₁₀ carboxylicacids, especially C₄-C₆ carboxylic acids, or straight-chain or branchedhalocarboxylic acids. Component VII can also comprise benzoic acid oraliphatic carboxylic acids with aromatic substituents, especiallyphenylacetic acid. Component VII is with particular preference selectedfrom acetic, propionic, n-butyric, isobutyric and n-valeric acid, inparticular from propionic, 3-chloropropionic, n-butyric and isobutyricacid.

Component VIII is preferably selected from unsaturated C₃-C₁₂ mono- ordicarboxylic acids or monoesters of such dicarboxylic acids, especiallyfrom α,β-ethylenically unsaturated C₃-C₆ mono- or dicarboxylic acids ormonoesters of the dicarboxylic acids.

Component VIII of the mixed esters that are employable is withparticular preference selected from acrylic, methacrylic, crotonic,vinylacetic, maleic, fumaric and undecenoic acid, especially fromacrylic and methacrylic acid.

Component VI is preferably esterified with component VII and VIII in anaverage molar degree of substitution of from 1.5 to 3, in particularfrom 1.6 to 2.7 and, with particular preference, from 2.3 to 2.6.Preferably about 1 to 30%, in particular from 1 to 20% or 1 to 10% and,with particular preference, from about 5 to 7% of the OH groups ofcomponent VI are esterified with component VIII.

The proportion of component VII to component VIII determines the hue ofthe polymer.

Highly suitable polymers of group c), moreover, are thepropargyl-terminated cholesteric polyesters or polycarbonates describedin DE-A-197 17 371.

It is also possible to employ crosslinkable oligo- orpolyorganosiloxanes, as are known, for example, from EP-A-0 358 208,DE-A-195 41 820 or DE-A-196 19 460.

Among these compounds preference is given to polyesters orpolycarbonates having at least one propargyl end group of the formulaR³C═C—CH₂—, where R³ is H, C₁-C₄-alkyl, aryl or Ar—C₁-C₄-alkyl (eg.benzyl or phenethyl) and which is attached to the polyesters orpolycarbonates directly or via a linker. The linker is preferablyselected from

—O—, —S—, —NR⁴—,

 (the propargyl group is attached to X),

where R⁴ is H, C₁-C₄-alkyl or phehyl, X is O, S or NR² and R² is H,C₁-C₄-alkyl or phenyl.

In the polyesters, the propargyl end group is preferably attached by wayof

The polyesters preferably comprise

(IX) at least one aromatic or araliphatic dicarboxylic acid unit and/orat least one aromatic or araliphatic hydroxycarboxylic acid unit, and

(X) at least one diol unit.

Preferred dicarboxylic acid units are those of the formula

especially those of the formula

where each of the phenyls or the naphthyl can carry 1, 2 or 3substituents selected independently from C₁-C₄-alkyl, C₁-C₄-alkoxy,halogen or phehyl, and where

W is NR, S, O, (CH₂)_(p)O(CH₂)_(q), (CH₂)_(m) or a single bond,

R is alkyl or hydrogen,

m is an integer from 1 to 15, and

p and q independently are integers from 0 to 10.

Preferred hydroxycarboxylic acid units are those of the formula

where each phenyl or the naphthyl can carry 1, 2 or 3 substituentsselected independently from C₁-C₄-alkyl, C₁-C₄-alkoxy, halogen orphenyl.

Preferred diol units are those of the formula

especially those of the formula

where

L is alkyl, alkoxy, halogen, COOR, OCOR, CONHR or NHCOR,

X is S, O, N, CH₂ or a single bond,

A is a single bond, (CH₂)_(n), O(CH₂)_(n), S(CH₂)_(n), NR(CH₂)_(n),

R is alkyl or hydrogen,

R¹ is hydrogen, halogen, alkyl or phehyl, and

n is an integer from 1 to 15.

Preference is given to polyesters comprising at least one dicarboxylicacid unit of the formula

and at least one diol unit of the formula

where R³ is H, halogen, C₁-C₄-alkyl, especially CH₃ or C(CH₃)₃, or isphenyl.

Further preferred compounds are diesters of the formulaP—Y—B—CO—O—A—O—CO—B—Y—P, where P is a propargyl end group of theabove-defined formula, Y is O, S or NR² (R²=C₁-C₄-alkyl), B is

where each phenyl or the naphthyl can carry 1, 2 or 3 substituentsselected independently from C₁-C₄-alkyl, C₁-C₄-alkoxy, halogen orphehyl, and A (together with the adjacent oxygens) is one of theabovementioned diol units.

Particularly preferred diesters are those of the above formula in whichB is

and especially diesters of the formula

HC≡CCH₂O—B—CO—O—A—O—CO—B—OCH₂—C≡CH, where

A is as defined under XI.

Further preferred compounds are polycarbonates comprising at least oneincorporated diol unit of the above formulae,

especially of the formulae

Preference is given here to those polycarbonates which comprise as diolunits at least one mesogenic unit of the formula

and at least one chiral unit of the formula

and, if desired, a nonchiral unit of the formula

where R¹ is as defined above and in particular is H or CH₃.

Particularly preferred polycarbonates are those having propargyl endgroups of the formula HC≡CCH₂O—R⁵—CO, in which R⁵ is

Further suitable polymers of group c) are cholesteric polycarbonatescontaining photoreactive groups even in a nonterminal position. Suchpolycarbonates are described in DE-A-196 31 658 and are preferably ofthe formula XIII

where the molar ratio w/x/y/z is from about 1 to 20/from about 1 to5/from about 0 to 10/from about 0 to 10. Particular preference is givento a molar ratio w/x/y/z of from about 1 to 5/from about 1 to 2/fromabout 0 to 5/from about 0 to 5.

In the formula XIII

A is a mesogenic group of the formula

B is a chiral group of the formula

D is a photoreactive group of the formula

and

E is a further, nonchiral group of the formula

where

L is alkyl, alkoxy, halogen, COOR, OCOR, CONHR or NHCOR,

X is S, O, N, CH₂ or a single bond,

R is alkyl or hydrogen,

A is a single bond, (CH₂)_(n), O(CH₂)_(n), S(Cu₂)_(n), NR(CH₂)_(n),

R¹ is hydrogen, halogen, alkyl or phehyl, and

n is an integer from 1 to 15.

If R¹ is alkyl or halogen and A is a single bond or if R¹ is H or alkyland A is

—O(CH₂)_(n)—, —S(CH₂)_(n) or —NR(CH₂)_(n)

the groups concerned are solubility-enhancing groups. Examples of theseare

Isosorbide, isomannide and/or isoidide is the preferred chiralcomponent.

The proportion of the chiral diol structural units is preferably withinthe range from 1 to 80 mol-% of the overall content of diol structuralunits, with particular preference from 2 to 20 mol-%, depending on thedesired interference hue.

Suitable polymers of group e) are chiral nematic polyesters havingflexible chains and comprising isosorbide, isomannide and/or isoidideunits, preferably isosorbide units, and also comprising at least onechain-flexibilizing unit selected from (and derived from)

(a) aliphatic dicarboxylic acids,

(b) aromatic dicarboxylic acids with a flexible spacer,

(c) α,ω-alkanoids,

(d) diphenols with a flexible spacer, and

(e) condensation products of a polyalkylene terephthalate orpolyalkylene naphthalate with an acylated diphenol and with an acylatedisosorbide,

as are described in DE-A-197 04 506.

The polyesters are noncrystalline and form stable Grandjean textureswhich can be frozen in on cooling to below the glass transitiontemperature. The glass transition temperatures of the polyesters are inturn, despite the flexibilization, above 80° C., preferably above 90° C.and, in particular, above 100° C.

The polyesters that can be employed include as units (a) preferablythose of the formula

—OC—(CH₂)_(n)—CO—

where n is from 3 to 15, especially 4 to 12, and with particularpreference adipic acid;

as units (b) preferably those of the formula

where

A is (CH₂)_(n), O(CH₂)_(n)O or (CH₂)_(o)—O—(CH₂)_(p),

n is from 3 to 15, especially 4 to 12, with particular preference 4 to10, and

o and p independently are from 1 to 7;

as units (c) preferably those of the formula

—O—4(CH₂)_(n)—O— or —O—(CH₂—CH₂—O)_(m)—,

where

n is from 3 to 15, especially 4 to 12, with particular preference 4 to10, and

m is from 1 to 10; and

as units (d) preferably those of the formula

where

A is (CH₂)_(n), O(CH₂)_(n)O or (CH₂)_(o)—O—(CH₂)_(p),

n is from 3 to 15, especially 4 to 12, with particular preference 4 to10, and

o and p independently are from 1 to 7.

The polyesters that can be employed additionally comprise, asnonflexible acid component, preferably dicarboxylic acid units of theformula

and as nonflexible alcohol component diol units of the formula

where

L is alkyl, alkoxy, halogen, COOR, OCOR, CONHR or NHCOR,

X is S, O, N, CH₂ or a single bond,

A is a single bond,

where

R¹ is hydrogen, halogen, alkyl or phenyl and

R is alkyl or hydrogen.

If desired, the polyesters that can be employed comprise additionalflexible diol units of the formula

where

R¹ is hydrogen, halogen, alkyl or phehyl,

A is (CH₂)_(n), O(CH₂)_(n), S(CH₂)_(n) or NR(CH₂)_(n), and

n is from 1 to 15.

Examples of preferred polymers of group d) are crosslinkable cholestericcopolyisocyanates as described in U.S. Pat. No. 8,834,745. Suchcopolyisocyanates feature repeating units of the formulae

and if desired of the formula

where

R¹ is a chiral aliphatic or aromatic radical,

R² is a crosslinkable radical and

R³ is an achiral radical.

Unless stated otherwise, alkyl is to be understood here (both alone andin definitions such as alkoxy, dialkyl, alkylthio, etc.) as branched andunbranched C₁-C₁₂-alkyl, preferably C₃-C₁₂—, with particular preferenceC₄-C₁₀- and, in particular, C₆-C₁₀-alkyl.

R¹ is preferably selected from (chiral) branched or unbranched alkyl,alkoxyalkyl, alkylthioalkyl, cycloalkyl, alkylphenyl or C₃-C₉-epoxyalkylradicals or radicals from esters of C₁-C₆ fatty acids withC₁-C₆-alkanols or C₃-C₉-dialkyl ketones. The ester radical may beattached to the nitrogen either via the fatty acid moiety or via thealkanol residue. The radical R¹ may have 1, 2 or 3 substituents, whichare identical or different and are selected from alkoxy,di-C₁-C₄-alkylamino, CN or C₁-C₄-alkylthio groups or halogen atoms.

R¹ is preferably selected from alkyl, alkoxyalkyl, radicals from estersof C₁-C₆ fatty acids with C₁-C₆-alkanols, C₃-C₉-dialkyl ketones andepoxidized C₃-C₉-epoxyalkyl radicals, where R¹ may be substituted by 1or 2 radicals which are identical or different and are selected fromalkoxy, halogen, CN and CF₃. Preferred substituents of branched orunbranched alkyl or alkoxy radicals are selected from alkoxy groups,halogen atoms and CN; for esters of C₁-C₆ fatty acids withC₁-C₆-alkanols, from alkoxy groups, halogen atoms, CN and CF₃; and, forC₃-C₉-dialkyl ketones, from alkoxy groups, halogen atoms and CN.

The main chain of the radical R¹ has, in particular, a length of from 3to 12, especially 6 to 10, preferably 6 to 8 members (carbons, oxygensand/or sulfurs). Particularly preferred radicals R¹ are selected from

With very particular preference, component III of the copolyisocyanatesthat can be employed is derived from 2,6-dimethylheptyl isocyanate.

The radical R² of the copolyisocyanates that can be employed ispreferably selected from C₃-C₁₁-alkenyl radicals, C₄-C₁₁-vinyl etherradicals (=vinyl C₂-C₉-alkyl ethers), ethylenically unsaturated C₃-C₁₁carboxylic acid radicals and esters of ethylenically unsaturated C₃-C₆monocarboxylic acids with C₂-C₆-alkanols, the linkage to the nitrogentaking place via the alkanol residue of the ester. The radical is withparticular preference selected from methyl acrylate, ethyl acrylate,propyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexylacrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate,n-butyl methacrylate, isobutyl methacrylate, and 2-ethylhexylmethacrylate, especially from ethyl acrylate or ethyl methacrylate.

The radical R³ preferably has the same definitions as the radical R¹.However, it is achiral, i.e. it has no center of chirality or is in theform of a racemic mixture.

With particular preference, the main chain of the radical R³ has alength of from 4 to 12, in particular 6 to 10, preferably 6 to 8 members(carbons, oxygens and/or sulfurs). Component V of the copolyisocyanatesof the invention is, with very particular preference, derived fromn-hexyl isocyanate, n-heptyl isocyanate or n-octyl isocyanate.

Components III, IV and V are preferably present in a molar ratioIII:IV:V of about 1 to 20:1 to 20:50 to 98, in particular about 5 to15:5 to 15:65 to 90, and, with particular preference, about 15:10:75.

The units III, IV and V can be distributed randomly in thecopolyisocyanates which can be employed.

Very particular preference is given, in accordance with the invention,to the presence in A¹ and A² of chiral compounds and nematic monomers ofgroup b), especially of chiral compounds of the formula 2:

or of the formula 5:

and nematic monomers of the formula 1:

or preferably of the formula 3:

or with particular preference of the formula 4:

in the cured state, where n₁, and n₂ in formulae 1 and 3 areindependently 4 or 6, R′ in formula 1 is H or Cl and the monomers of theformula 1 or 3 are preferably employed as mixtures of the compounds withn₁/n₂=4/4, 4/6, 6/4 or 6/6, and R in formula 4 is H, Cl or CH₃. It isalso possible in accordance with the invention, however, for othercholesteric mixtures, examples being the mixtures disclosed in EP-A-686674, to be present in the cured state in A¹ and A².

The cholesteric mixtures, and the formulations comprising the absorptionpigment, can be diluted with any suitable diluent before being appliedto the carrier.

Diluents which can be employed in the process of the invention for thecompounds of the groups a) and b) are linear or branched esters,especially acetic esters, cyclic ethers and esters, alcohols, lactones,aliphatic and aromatic hydrocarbons, such as toluene, xylene andcyclohexane, and also ketones, amides, N-alkylpyrrolidones, especiallyN-methylpyrrolidone, and in particular tetrahydrofuran (THF), dioxaneand methyl ethyl ketone (MEK).

Examples of suitable diluents for the polymers of group c) are ethersand cyclic ethers, such as tetrahydrofuran or dioxane, chlorinatedhydrocarbons, such as dichloromethane, 1,1,2,2-tetrachloroethane,1-chloronaphthalene, chlorobenzene or 1,2-dichlorobenzene. Thesediluents are particularly suitable for polyesters and polycarbonates.Examples of suitable diluents for cellulose derivatives are ethers, suchas dioxane, or ketones, such as acetone. If copolyisocyanates areemployed as polymers of group d) it is advisable to use polymerizablediluents as described in U.S. Pat. No. 8,834,745. Examples of suchpolymerizable diluents are

esters of α,β-unsaturated mono- or dicarboxylic acids, especially C₃-C₆mono- or dicarboxylic acids, with C₁-C₁₂-alkanols, C₂-C₁₂-alkanediols ortheir C₁-C₆-alkyl ethers and phenyl ethers, for example the acrylatesand methacrylates, hydroxyethyl or hydroxypropyl acrylate ormethacrylate, and 2-ethoxyethyl acrylate or methacrylate;

vinyl C₁-C₁₂-alkyl ethers, such as vinyl ethyl¹, vinyl hexyl or vinyloctyl ether;

vinyl esters of C₁-C₁₂ carboxylic acids, such as vinyl acetate, vinylpropionate, vinyl laurate;

C₃-C₉ epoxides, such as 1,2-butylene oxide, styrene oxide;

N-vinylpyrrolidone, N-vinylcaprolactam, N-vinylformamide;

vinylaromatic compounds, such as styrene, α-methylstyrene,chlorostyrene, and

compounds having two or more crosslinkable groups, such as diesters ofdiols (including polyethylene glycols) with acrylic or methacrylic acid,or divinylbenzene.

Examples of preferred polymerizable diluents are 2-ethoxyethyl acrylate,diethylene glycol diacrylate, ethylene glycol dimethacrylate, diethyleneglycol dimethacrylate, triethylene glycol dimethacrylate, diethyleneglycol monomethyl ether acrylate, phenoxyethyl acrylate andtetraethylene glycol dimethacrylate. A particularly preferredpolymerizable diluent is styrene.

The mixtures of groups a), b) or c) may also include, in small amounts,polymerizable diluents in addition to the inert diluent. Preferredpolymerizable solvents which can be added to a), b) or c) are acrylates,especially acrylates of relatively high functionality such as bis-,tris- or tetraacrylates, and with particular preference high-boilingoligoacrylates. The preferred amount added is approximately 5% byweight, based on the overall weight of the mixture.

Water may also be added to the diluent or even employed as the diluentalone.

For photochemical polymerization, the cholesteric mixture may includecustomary commercial photoinitiators. For curing by electron beams, suchinitiators are not required. Examples of suitable photoinitiators areisobutyl benzoin ether, 2,4,6-trimethylbenzoyldiphenylphosphine oxide,1-hydroxycyclohexyl phenyl ketone,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)furan-1-one, mixtures ofbenzophenone and 1-hydroxycyclohexyl phenyl ketone,2,2-dimethoxy-2-phenylacetophenone, perfluorinated diphenyltitanocenes,2-methyl-1-(4-[methylthio]-phenyl)-2-(4-morpholinyl)-1-propanone,2-hydroxy-2-methyl-1-phenylpropan-1-one, 4-(2-hydroxyethoxy)phenyl2-hydroxy-2-propyl ketone, 2,2-diethoxyacetophenone,4-benzoyl-4′-methyl-diphenyl sulfide, ethyl 4-(dimethylamino)benzoate,mixtures of 2-isopropylthioxanthone and 4-isopropylthioxanthone,2-(dimethylamino)ethyl benzoate, d,l-camphorquinone,ethyl-d,l-camphorquinone, mixtures of benzophenone and4-methylbenzophenone, benzophenone, 4,4′-bisdimethylaminobenzophenone,(η⁵-cyclopentadienyl)(η⁶-isopropylphenyl)iron(II) hexafluorophosphate,triphenylsulfonium hexafluorophosphate or mixtures of triphenylsulfoniumsalts, and also butanediol diacrylate, dipropylene glycol diacrylate,hexanediol diacrylate, 4-(1,1-dimethylethyl)cyclohexyl acrylate,trimethylolpropane triacrylate and tripropylene glycol diacrylate.

The brightness of the cholesteric layers A¹ and A² can be increased byadding small amounts of suitable leveling agents. It is possible toemploy from about 0.005 to 1% by weight, in particular from 0.01 to 0.5%by weight, based on the amount of cholesteric employed. Examples ofsuitable leveling agents are glycols, silicone oils and, in particular,acrylate polymers, such as the acrylate copolymers obtainable under thename Byk 361 or Byk 358 from Byk-Chemie and the modified, silicone-freeacrylate polymers obtainable under the name Tego flow ZFS 460 from Tego.

The cholesteric mixture may also include stabilizers to counter theeffects of UV and weather. Examples of suitable such additives arederivatives of 2,4-dihydroxybenzophenone, derivatives of2-cyano-3,3-diphenyl acrylate, derivatives of2,2′,4,4′-tetrahydroxybenzophenone, derivatives ofortho-hydroxyphenylbenzotriazole, salicylic esters,ortho-hydroxyphenyl-S-triazines or sterically hindered amines. Thesesubstances can be employed alone or, preferably, as mixtures.

The present invention additionally provides a process for producing apigment of the invention, which comprises applying the layers A¹, B andA² atop one another to a substrate, simultaneously or with a timedifferential, curing the layers by heat, UV radiation, electron beams orby rapid cooling to below the glass transition temperature,simultaneously or with a time differential, removing the fully curedlayers from the substrate, and then comminuting them to give pigments.

The application of the layers A¹, B and A² to the substrate can becarried out by means of customary techniques selected, for example, fromair-knife coating, knife coating, air blade coating, squeeze coating,impregnation, reverse roll coating, transfer roll coating, gravurecoating, kiss coating, casting, spray coating, spin coating or printingtechniques, such as letterpress (relief), intaglio, flexographic, offsetor screen printing. The layers A¹, B and A² are preferably applied tothe substrate by means of casting or offset printing.

The substrate is preferably mobile and with particular preference is amoving substrate in strip form.

Suitable layer substrates are, preferably, known films formed frompolyesters, such as polyethylene terephthalate or polyethylenenaphthalate, and also polyolefins, cellulose triacetate, polycarbonates,polyamides, polyimides, polyamidoimides, polysulfones, aramids oraromatic polyamides. The thickness of the layer substrates is preferablyfrom about 5 to 100 μm, in particular from about 10 to 20 μm. The layersubstrate can be subjected beforehand to a corona discharge treatment, aplasma treatment, a gentle adhesion treatment, a heat treatment, adedusting treatment or the like. The layer substrate preferably has amean center-line surface roughness of 0.03 μm or less, in particular of0.02 μm or less, and, with particular preference, 0.01 μm or less. It isis desirable, moreover, for the substrate to have not only a low meancenter-line surface roughness of this kind but also to possess no greatprojections (raised areas) of 1 μm or more. The roughness profile of thesurface of the substrate can be varied by means of fillers which areadded to the layer substrate in the course of its production. Examplesof suitable fillers are oxides and carbonates of Ca, Si and Ti, and fineorganic powders of acrylic substances.

The substrate can also be a metallized foil or a preferably polishedmetal strip.

The layers A¹, B and A² can be of low or high viscosity, but arepreferably of low viscosity, when they are applied to the substrate. Forthis purpose, the cholesteric mixtures, or the formulations comprisingabsorption pigment, can be applied to the carrier in indiluted orminimally diluted form at elevated temperature or in highly diluted format a low temperature. It is particularly preferred to apply the threelayers A¹, B and A² wet-on-wet to the carrier in one operation, then todry them together, if appropriate, and subsequently to subject them toconjoint curing.

For the simultaneous application wet-on-wet of said layers it isparticularly preferred if the layer B comprises absorption pigmentsbound in a matrix of the same cholesteric mixture as is also present inthe layers A¹ and A². By this means, possibly disruptive layerboundaries between A¹, B and A² are avoided, so giving a homogeneoussystem which comprises homogenously distributed pigments in the centralregion.

Casting techniques are particularly suitable for the simultaneousapplication of said layers, especially knife or bar coating processes,cast-film extrusion or stripping processes, and the cascade coatingprocess.

In the case of the knife or bar coating process, the liquid is appliedto a substrate through a slot in a casting block, the layer thicknessbeing adjustable by way of a defined knife or bar gap between a roller,over which the carrier is guided, and the lip of the coater. To applythe bottom (first) layer, the first casting block is brought toward theroller; to apply the second layer, a second casting block is broughttoward the first casting block and, to apply the third layer, a thirdcasting block is brought against the second. An analogous process isdescribed in DE-A-19 504 930, which is incorporated herein by reference.All three liquids run to their respective coating blade or bar and arecoated out simultaneously over one another.

In the case of the cast-film extrusion or stripping process, a flexiblesubstrate, such as a film, is guided past the coater head under definedweb tension between two rollers. The amounts of liquid appropriate tothe desired layer thickness are applied simultaneously to the substratefrom three parallel casting slots arranged transverse to the runningdirection of the web. A process of this kind is described, for example,in EP-A 431 630, DE-A-3 733 031 and EP-A-452 959, which are incorporatedherein by reference.

In the cascade coating process, the substrate is guided over a roller.The liquids to be applied run over one another from differently arrangedslots and then run together on the carrier. This process is likewisedescribed in DE-A-19 504 930.

It is of course also possible first to apply only one cholesteric layer,to subject this layer, if desired, to drying and to curing, and then toapply two layers wet-on-wet to the cured cholesteric layer by means, forexample, of one of the abovementioned processes. It is likewise possibleto subject each layer to individual and successive application, optionaldrying and curing.

If casting techniques are employed, the pourable cholesteric mixturepreferably has a viscosity in the range from about 10 to 500 mPas, inparticular from about 10 to 100 mpas, measured at 23° C. The cholestericmixture is, with particular preference, applied to the substrate at arate from about 1 to 800 m/min, in particular from about 5 to 100 m/min.It is preferred to use a casting apparatus whose casting slot width isin the range from about 2 to 50 μm, in particular from about 4 to 15 μm.The cholesteric mixture is preferably applied under elevated pressure,in particular at a coater overpressure in the range from about 0.01 to0.7 bar, with particular preference from 0.05 to 0.3 bar.

The cured layers can be removed from the substrate, for example, byguiding the substrate over a deflecting roller having a small diameter.As a consequence of this the crosslinked material then peels away fromthe substrate. Other known methods are equally suitable: for example,the stripping of the substrate over a sharp edge, or by way of an airknife, ultrasound or combinations thereof. The cholesteric material, nowdevoid of its substrate, is comminuted to a desired particle size. Thiscan be done, for example, by grinding in universal mills. In order tonarrow the particle size distribution the comminuted pigments cansubsequently be classified by means, for example, of a sieving process.

The invention additionally provides compositions comprising pigments ofthe invention.

Particularly preferred compositions of the invention are coatingmaterials, such as paints and varnishes, which comprise not only thepigments of the invention but also one or more substances selected fromwaterborne coatings, for example in the form of aqueous dispersions,such as PMA, SA, polyvinyl derivatives, PVC, polyvinylidene chloride, SBcopolymer, PV-AC copolymer resins, or in the form of water-solublebinders, such as shellac, maleic resins, rosin-modified phenolic resins,linear and branched, saturated polyesters, amino resin-crosslinkingsaturated polyesters, fatty acid-modified alkyd resins, plasticized urearesins, or in the form of water-thinnable binders, such as PUdispersions, EP resins, urea resins, melamine resins, phenolic resins,alkyd resins, alkyd resin emulsions, silicone resin emulsions; powdercoatings, such as powder coatings for TRIBO/ES, such as polyestercoating powder resins, PU coating powder resins, EP coating powderresins, EP/SP hybrid coating powder resins, PMA coating powder resins,or powder coatings for fluidized-bed sintering, such asthermoplasticized EPS, LD-PE, LLD-PE, HD-PE; solventborne coatings, suchas one- and two-component coating materials (binders) examples beingshellac, rosin esters, maleate resins, nitrocelluloses, rosin-modifiedphenolic resins, physically drying saturated polyesters, aminoresin-crosslinking saturated polyesters, isocyanate-crosslinkingsaturated polyesters, self-crosslinking saturated polyesters, alkydswith saturated fatty acids, linseed oil alkyd resins, soya oil resins,sunflower oil alkyd resins, safflower oil alkyd resins, ricinene alkydresins, tung oil/linseed oil alkyd resins, mixed-oil alkyd resins,resin-modified alkyd resins, styrene/vinyltoluene-modified alkyd resins,acrylicized alkyd resins, urethane-modified alkyd resins,silicone-modified alkyd resins, epoxy-modified alkyd resins, isophthalicacid alkyd resins, unplasticized urea resins, plasticized urea resins,melamine resins, polyvinyl acetals, noncrosslinking P(M)A homo- orcopolymers, noncrosslinking P(M)A homo- or copolymers with nonacrylicmonomers, self-crosslinking P(M)A homo- or copolymers, P(M)A copolymerswith other nonacrylic monomers, externally crosslinking P(M)A homo- orcopolymers, externally crosslinking P(M)A copolymers with nonacrylicmonomers, acrylate copolymer resins, unsaturated hydrocarbon resins,organic-soluble cellulose compounds, silicone combination resins, PUresins, P resins, peroxide-curing unsaturated synthetic resins,radiation-curing synthetic resins, both photoinitiator-containing andphotoinitiator-free radiation-curing synthetic resins; solvent-freecoating materials (binders) such as isocyanate-crosslinking saturatedpolyesters, two-pack PU resin systems, moisture-curing 1-component PUresin systems, EP resins, and also synthetic resins—individually or incombination—such as acrylonitrile-butadiene-styrene-copolymers, BS,cellulose acetate, cellulose acetobutyrate, cellulose acetopropionatecellulose nitrate, cellulose propionate, artificial horn, epoxy resins,polyamide, polycarbonate, polyethylene, polybutylene terephthalate,polyethylene terephthalate, polymethyl methacrylate, polypropylene,polystyrene, polytetrafluoro-ethylene, polyvinyl chloride,polyvinylidene chloride, polyurethane, styrene-acrylonitrile copolymers,or unsaturated polyester resins in the form of granules, powders orcasting resin.

The compositions of the invention may additionally comprise stabilizersto counter the effects of UV and weather, and also inorganic or organicpigments, as described above.

The pigments of the invention can be incorporated individually or inmixtures into the compositions of the invention where they may ifdesired be subjected to additional alignment by methods which initiateshear forces. Suitable methods of aligning the pigments of the inventionare printing and knife coating or, in the case of magnetic pigments,applying an external magnetic field.

The present invention additionally provides coating materials comprisingat least one multilayer pigment of the invention, preferably coatingmaterials selected from effect paints, effect inks or effect films, andespecially from self-opacifying effect paints, inks or films.

The present invention also provides for the use of the pigments of theinvention in the vehicle and vehicle accessories sector, in the EDT, inthe leisure, sport and games sector, as an optical component such as apolarizer or filter in the cosmetics field, in the textile, leather orjewelry field, in the gift product field, in writing utensils, packagingor spectacle frames, in the construction sector, in the household sectorand in connection with printed products of all kinds, such as cardboardpackaging, other packaging materials, carrier bags, papers, labels orsheets, or for preparing inks and paints.

The color effects which can be achieved by means of the cholestericpigments of the invention embrace—owing to the host of achievablereflection wavelengths—the UV and IR region as well as, of course, theregion of visible light. If the pigments of the invention are applied toor incorporated into bank notes, cheque cards, other cashless means ofpayment or ID (by means, for example, of known printing techniques),this considerably hinders the identical copying, and especially thecounterfeiting, of these articles. The present invention thereforeadditionally provides for the use of the pigments of the invention forthe anticounterfeiting treatment of articles, especially bank notes,cheque cards or other cashless means of payment or ID.

Also provided for by the present invention is the use of thecompositions of the invention for coating articles of utility and forpainting vehicles.

The invention will now be illustrated on the basis of the followingworking examples and with reference to attached

FIG. 1. Here, FIG. 1 shows the diagrammatic representation of a coatingapparatus which can be employed in accordance with the invention.

EXAMPLE 1 Preparing Three-layer Pigments with a Pigmented Absorber LayerComprising a Cholesteric Radiation-curable Binder

a) Preparing the 1st Cholesteric Layer

A cholesteric mixture of the above-described group b) was employed,which comprised a compound of the above formula 2 as chiral monomer anda mixture of compounds of the above formula 3 as achiral, nematicmonomer. The undiluted cholesteric mixture contained 90.5% by weight ofthe achiral, nematic compound, 6.5% by weight of the chiral compoundand, as photoinitiator, 3% by weight of 1-hydroxycyclohexyl phenylketone, which is marketed under the name Irgacure 184. The solvent usedwas methyl ethyl ketone.

Coating was carried out with a coating apparatus which is representeddiagrammatically in FIG. 1. A glossy black polyethylene terephthalatefilm (PET film) (G) already coated on its reverse face and having athickness of 15 μm was unrolled continuously from the film winder (F)and was coated with a blade coater. The thickness of the cholestericlayer was 2 μm. Drying took place at 60° C. in the dryer (C). The layerwas cured by UV fixing in the UV unit (A), while the dried strip wasguided over the cooling roll (B). The cured cholesteric layer was woundup onto the roller (D).

The reflection maximum of the layer was at 520 nm.

b) Preparing an Absorber Layer Pigmented with Carbon Black, with aCholesteric Binder

150 g of pigment-grade carbon black Regal 400R (manufacturer: CabotCorporation) are kneaded for 1 hour with 3 g of stearic acid, 80 g of aphosphonate-functional dispersing resin (50% strength intetrahydrofuran, described in DE-A-195 16 784) and 40 g of methyl ethylketone in a laboratory compounder having a capacity of 300 ml. Theresulting kneaded mass (solids content 70.7%) is subsequently adjustedto a solids content of 25% in a dissolver, using 499 g of methyl ethylketone. This dispersion is then fully dispersed to the optimum extent ina stirred mill (model Dispermat SL, milling chamber volume 125 ml,grinding media zirconium oxide 1-1.25 mm). The progress of dispersionduring this step is monitored by means of an interference contrasttechnique (EP-B-0-032-710). The ultimate fineness is achieved when thesurface to be tested is agglomerate-free. A layer prepared from thisdispersion is highly glossy and has a base roughness of ≦100 nm. 500 gof a 60% strength cholesteric solution together with 0.3 g of Byk 361(Byk-Chemie) are mixed thoroughly into the resultant dispersion for 30minutes using a dissolver. Following the stirred incorporation of 9 g ofphotoinitiator Irgacure 907 (Ciba Geigy) this dispersion (solids content39.2%) can be applied. For this purpose, it is applied in a layerthickness of 0.8 μm (dry thickness) to the 1st cholesteric layer inanalogy to step a), is physically dried, and then is radiation-curedunder a nitrogen atmosphere.

c) Preparing the 2nd Cholesteric Layer

A further cholesteric layer is applied in analogy to step a) to thecured absorber layer, and is dried and cured. In terms of itscomposition and layer thickness, the 2nd cholesteric layer is comparablewith the 1st cholesteric layer.

d) Preparing Pigments

The cured three-layer assembly is removed from the carrier film by beingdamaged with a razor blade transverse to the film web direction and thenblown off with compressed air, which is injected through a slot die. Inthe course of this procedure, the coated film is guided continuouslypast the slot die and the three-layer assembly removed by blowing iscollected in the form of flakes. The three-layer flakes are 6 μm thickand on both sides, when viewed straight on, show a strong green colorwith a color change to blue when the flakes are viewed at an obliqueangle.

10 g of cholesteric flakes are mixed with 100 g of sodium chloride andthe mixture is milled 6 times for 2 minutes in a beater blade mill.After milling, the salt is washed out with water and the pigment isisolated.

The pigment prepared in this way possesses a high brightness on bothsides with a coloredness which is dependent on the viewing angle.

EXAMPLE 2 Preparing Cholesteric Three-layer Pigments without aCholesteric Interlayer

a) Preparing the 1st Cholesteric Layer

With the aid of a coater apparatus as in Example 1, a solutionconsisting of 45 parts of cholesteric mixture (96.2% nematic componentof formula (3) and 3.8% chiral component of formula (5)), 3 parts ofphotoinitiator Irgacure® 907 (from Ciba-Geigy), 0.1 part of Byk 361(from Byk) and 51.9 parts of methyl ethyl ketone are applied to apolyester film. The coated film is then passed through a drying tunnelwhich is thermostated at 60° C. Subsequently, the physically dried layeris cured in-line by radiation with UV light under a nitrogen atmosphere,and the coated film is wound up onto a spool. The cholesteric layer hasa thickness of 2 μm and, vertically to the plane of the layer, reflectslight, with a reflection maximum at a wavelength of 505 nm. When viewedwith the eye, the layer appears green when viewed straight on with acolor change to blue when the layer is viewed at an oblique angle.

b) Preparing the Interlayer

The film coated as described under a) is coated, using the same coaterapparatus, with a solution consisting of 70.82 parts of black printingink Flexoplastol VA/2-black (from BASF AG), 7.08 parts of reactivediluent (triacrylate of propoxylated/ethoxylated trimethylolpropane;Laromer® PO 33F from BASF AG), 0.85 part of photoinitiator Irgacure® 500(from Ciba-Geigy), and 21.25 parts of tetrahydrofuran. The coated filmis then passed through a drying tunnel which is thermostated to 60° C.,and the film is physically dried. Subsequently, the layer is curedin-line by radiation with UV light under a nitrogen atmosphere and thecoated film is wound up onto a spool. The black layer has a thickness of1 μm.

c) Preparing the 2nd Cholesteric Layer

The film coated as described under a) and b) is coated, using the samecoater apparatus, with a solution consisting of 45 parts of cholestericmixture (cf. step a)), 3 parts of photoinitiator Irgacure® 907, 0.1 partof Byk 361 and 51.9 parts of methyl ethyl ketone. The coated film isthen passed through a drying tunnel which is thermostated at 60° C., andthe film is physically dried. Subsequently, the layer is cured in-lineby radiation with UV light under a nitrogen atmosphere and the coatedfilm is wound up onto a spool. The cholesteric layer has a thickness of2 μm and, vertically to the plane of the layer, reflects light, with areflection maximum at 510 nm. When viewed with the eye, the layerappears green when viewed straight on with a color change to blue whenthe layer is viewed at an oblique angle.

In accordance with the operating steps a), b) and c), a 3-layer assemblyis obtained on the polyester carrier film. The mechanical stability ofthe 3-layer assembly was determined by measuring the tear strength andthe peel force. For this purpose, the films coated with the 3-layerassembly were cut into 3.81 mm wide strips, the force transducer wasstuck to the three-layer assembly, and the force required to tear (tearstrength) or to peel (peel strength) the already torn layer is measured.The tear strength between the first cholesteric layer and the blackinterlayer is 0.065 N and the peel strength is 0.005 N.

d) Removing the Three-layer Assembly From the Carrier Film

The three-layer assembly described under a), b) and c) is removed fromthe polyester carrier film by damaging the three-layer structure with arazor blade transversely to the film web direction and then blowing itoff with compressed air, which is injected through a slot die. In thecourse of this operation, the coated film is guided continuously pastthe slot die and the three-layer assembly removed by blowing iscollected in the form of flakes. The three-layer flakes are 6 μm thickand on both sides, when viewed straight on, show a strong green colorwith a color change to blue when the flakes are viewed at an obliqueangle.

e) Milling the Three-layer Flakes to a Pigment

10 g of cholesteric flakes prepared as described under d) are mixed with100 g of sodium chloride and the mixture is milled 6 times for 2 minutesin a beater blade mill. After milling, the salt is washed off with waterand the pigment is isolated. In the course of milling, there is partialdelamination of the three-layer pigment.

EXAMPLE 3 Preparing Cholesteric Three-layer Pigments without aCholesteric Interlayer

a) First of All, the 1st Cholesteric Layer is Prepared as Described inExample 2 Under a).

b) Preparing the Interlayer

The film coated as described under a) is coated, using the same coaterapparatus, with a solution consisting of 28.33 parts of black printingink Flexoplastol VA/2-black (from BASF AG), 2.83 parts of reactivediluent (Laromer® PO 33F from BASF AG), 0.34 part of photoinitiatorIrgacure® 500 (from Ciba-Geigy), 60 parts of a 20 percent strengthsolution of a copolymer of ethylhexyl acrylate and acrylic acid;Acronal® 101 L from BASF AG) in tetrahydrofuran, and 8.5 parts oftetrahydrofuran. The coated film is then passed through a drying tunnelwhich is thermostated to 60° C., and the film is physically dried.Subsequently, the layer is cured in-line by radiation with UV lightunder a nitrogen atmosphere and the coated film is wound up onto aspool. The black layer has a thickness of 1 μm.

c) Preparing the 2nd Cholesteric Layer

The 2nd cholesteric layer is applied to the black interlayer asdescribed in Example 2 under c), to give a three-layer assembly. Thetest of the mechanical stability of the three-layer assembly gives atear strength between the 1st cholesteric layer and the black interlayerof 0.19 N and a peel force of 0.01 N.

d) Removing the Three-layer Assembly From the Carrier Film

The three-layer assembly is removed from the carrier film as describedin Example 2 under d).

e) Milling the Three-layer Flakes to a Pigment

Milling to a pigment takes place as described in Example 2 under e).Microscopic evaluation indicates a much smaller proportion than inExample 2 of delaminated pigment particles which are 6 μm thick and showa strong green color on both sides when viewed straight on. If theviewing angle is changed, then a color change from green to blue isobtained. When viewed against a white background, the pigment particlesare opaque.

We claim:
 1. A platelet-shaped cholesteric multilayer pigment whichcomprises the layer sequence A¹/B/A², wherein A¹ and A² are identical ordifferent and each comprise at least one cholesteric layer wherein thethickness of each cholesteric layer is about 0.5 to 2 μm, and B is atleast one interlayer which separates the layers A¹ and A² from oneanother and which absorbs all or some of the light transmitted by thelayers A¹ and A².
 2. A multilayer pigment as claimed in claim 1, whereinA¹ and A² possess identical or different optical properties.
 3. Amultilayer pigment as claimed in claim 2, wherein A¹ and A² reflectlight of identical or different wavelength and/or are of identical ordifferent handedness.
 4. A multilayer pigment as claimed in claim 1,wherein B comprises at least one organic or inorganic absorptionpigment, optionally bound in a binder matrix.
 5. A multilayer pigment asclaimed in claim 1, wherein A¹ and A² comprise cholesteric mixturesselected from the group consisting of a) at least one cholesteric,polymerizable monomer, b) at least one achiral, nematic, polymerizablemonomer and one chiral compound, c) at least one cholesteric,crosslinkable oligomer or polymer, d) a cholesteric polymer in apolymerizable diluent, and e) at least one cholesteric polymer whosecholesteric phase can be frozen in by rapid cooling to below the glasstransition temperature, in the cured state.
 6. A multilayer pigment asclaimed in claim 1, wherein B comprises a binder matrix comprising atleast one cholesteric mixture selected from the group consisting of a)at least one cholesteric, polymerizable monomer, b) at least oneachiral, nematic, polymerizable monomer and one chiral compound, c) atleast one cholesteric, crosslinkable oligomer or polymer, d) acholesteric polymer in a polymerizable diluent, and e) at least onecholesteric polymer whose cholesteric phase can be frozen in by rapidcooling to below the glass transition temperature, in the cured state.7. A multilayer pigment as claimed in claim 6, where A¹, B and A2comprise the same cholesteric mixtures.
 8. A process for producing amultilayer pigment as claimed in claim 1, which comprises applying thelayers A¹, B and A² atop one another to a substrate, simultaneously orwith a time differential, curing the layers, simultaneously or with atime differential, removing the fully cured layers from the substrateand then comminuting them to produce multilayer pigments.
 9. A processas claimed in claim 8, wherein the applied layers are dried prior tocuring.
 10. A process as claimed in claim 8, which comprises applyingthe layers A¹, B and A² to the substrate by means of a techniqueselected from the group consisting of air knife.
 11. A process asclaimed in claim 10, which comprises applying the layers A¹, B and A² tothe substrate by means of letterpress (relief), intaglio, flexographic,or offset or screen printing.
 12. A process as claimed in claim 10,wherein the layers A¹, B and A² are applied by means of casting oroffset printing.
 13. A composition comprising at least one multilayerpigment as claimed in claim
 1. 14. A coating material comprising atleast one multilayer pigment as claimed in claim 1.